Many gripping means are known to be used with various tools or for gripping, holding and moving objects, and particularly as a terminal organ of a manipulator or programmed robot. Most often, these grippers make broad surface contact with the objects to be gripped. Alternatively, smaller pads on fingers are used and an object is held by forcing the fingers and pads in an inward direction against the outer surface of the object.
In some instances grippers are provided with two fingers which do the gripping either while parallel to one another, or while pivoting about two shafts affixed to a base piece. These two fingers generally terminate in planar, in some cases resilient, contact surfaces, permitting ordinary objects to be held immobile by making broad surface or linear contact on each surface.
U.S. Pat. No. 4,653,793 in the name of Guinot et al. describes a gripper having a plurality of contact pads providing a point contact with an object to be gripped. In operation, when an object is to be gripped, the pads apply an inward force on the outside surface of the object.
Another robotic gripper is described in U.S. Pat. No. 5,178,431 in the name of Voellmer, the gripper including a pair of fingers movable toward and away from one another while the fingers remain parallel to each other.
U.S. Pat. No. 4,367,893 in the name of Berg and U.S. Pat. No. 4,828,276 in the names of Link et al. disclose grippers whose fingers only move in a plane, whose finger axes must be parallel to the surface of the part in order to hold it, and which withstand out-of-plane forces and moments by means of contact friction alone. In contrast to the present invention, these devices are unable to grasp parts whose surfaces are not parallel, or hold parts subject to large out-of-plane forces or moments (e.g. due to part weight).
U.S. Pat. No. 5,566,466 in the name of Heane and U.S. Pat. No. 5,669,652 in the name of Reising et al. describe grippers designed for holding flat sheet objects. Each device may only move its fingers in a plane and as a result is limited to holding flat, or nearly flat, sheet objects.
U.S. Pat. No. 5,161,847 in the name of Yakou describes a gripper with three plain, pin-like elongate cylindrical fingers. One finger has two degrees of mobility, while the remaining two fingers share four degrees of mobility, for a total of six degrees of mobility. To position each of the fingers of a three finger gripper anywhere in 3D space, as is required to provide the flexibility to hold many objects, requires a total of nine degrees of mobility. The device of Yakou has the limitation that in order to place the fingers at any point in space requires the use of three degrees of mobility from the robot holding the gripper. This is undesirable for at least two reasons. Firstly, not all robots may possess the needed degrees of freedom. Secondly, the larger mass carried by the robot's actuators will cause their movements to be slow when compared to a gripper's actuators.
In accordance with an aspect of this invention three fingers are provided each having three degrees of mobility X, Y and Z; this provides the ability to position each finger anywhere in 3D space only restricted by the size of the device; furthermore, the gripper in accordance with an aspect of the invention does not require any degrees of mobility from the robot. In fact, the gripper could be mounted on a stationary platform and still could be used to the pickup, hold, transport, and drop-off objects with a wide variety of shapes.
The gripper described by Yakou in U.S. Pat. No. 5,161,847 is further limited by the plain cylindrical fingers employed. As with the devices of Link et al. and of Berg, out-of-plane forces and moments are resisted by means of contact friction alone. For example, a typical application is shown in FIG. 11 of the instant invention, similar to FIG. 13 in Yakou. The object is subject to clamping forces F and frictional forces equal to the coefficient of friction (.mu.) between the finger and object surfaces times the clamping force. These forces are applied by the fingers and create an out-of-plane moment. Using Yakou's design the object will fail to be held if the ratio of d1 over d2 (i.e. the object's height over it's width) exceeds the coefficient of friction. If the fingers and object are made of steel, which is the typical situation, this ratio will be limited to approximately 0.2 if both surfaces are dry, and to approximately 0.05 if either is wet or oily. As a result, the device of Yakou is limited to holding fairly flat objects. It can also only hold an object subjected to out-of-plane forces less than twice the coefficient of friction times the clamping force.
The fingers described in the instant invention employ non-clamp means such as knurling or V grooves in place of the plain cylinders employed by the device of Yakou. Although grooves provided at the ends of fingers are shown by Hearne in U.S. Pat. No. 5,566,466, Hearne's design is limited to grasping flat objects. Notwithstanding, there is a significant unexpected advantage in providing fingers having multiple degrees of freedom having some sort of grooved means or no-clamp means at or near their ends. As shown in FIG. 12, one embodiment would be knurl the surfaces of the fingers in the general area where they would come in contact with an object, labelled K in the figure. The teeth of the knurled surface provide a much stronger constraint than that provided by frictional contact. A further embodiment of non-clamping means, namely the use of circumferential V grooves applied to the fingers' surfaces, is shown in FIG. 13, labelled G. It may be easily shown that the object will fail to be held if the ratio of d1 over d2 (i.e. the object's height over it's width) exceeds the sum of the coefficient of friction and the tangent of the groove angle .alpha.. For a typical groove angle of 45.degree., this ratio will be 1.2 (assuming dry steel on steel contact), so that objects ranging from completely flat to those which are more tall than wide may be held by the new invention. This is a substantial improvement over that of Yakou. Assuming the object can be held, the use of grooves allows theoretically infinite out-of-plane forces and moments to be resisted, limited only by the strength of the fingers, actuators, and the gripper's frame. Furthermore, this does not require the application of large clamping forces so that thin or fragile objects may be held. The surfaces of the object also need not be either parallel with the finger as with the device of Yakou or perpendicular to the finger as with the devices of Hearne and of Reising et al. allowing objects with more complex curved surfaces, for example as shown in FIG. 1 of the instant application, to be held.
Although some of the aforementioned prior art grippers may adequately provide their intended function, there is a need for a programmable gripper for accurately fixturing three-dimensional sheet metal parts of various shapes.
For example, in the automotive and aircraft industry, sheet metal parts must be fixtured during the assembly of vehicle bodies. Furthermore, it is preferred to have a single gripper that can easily be programmed to fixture parts having a plurality of different shapes and sizes.
It is therefore an object of this invention to provide a multi-degree-of-freedom gripper for use in robotic fixtureless assembly.
It is a further object of this invention to provide a gripper for fixturing a plurality of different sheet metal parts by only changing computer controlled axes.
It is a further object of the invention to provide a gripper that can grip in the absence of a robot.